Resistive switching in mixed conductors : Ag2S as a model system

Resistive switching memories have gained an increased interest due to the possibilities for downscaling of memory devices down to a few nanometers. These memories consist of a resistive material sandwiched between two metal electrodes, and applying a voltage between them induces resistance switching. In this thesis we study the specific case when switching is due to the reversible formation of a conductive path that connects and disconnects the electrodes. We investigate the electrical conductance properties and transport mechanisms in solid electrolyte memory devices, to gain a fundamental understanding of conductance switching. Our model system consists of Ag2S thin films with a Ag bottom electrode and a Pt AFM or STM tip as top electrode. We present a quantitative analysis of the steady state transport that leads up to resistance switching. We discuss the relation between stoichiometry and resistance changes in the material, and the necessity of Ag supersaturation prior to nucleation and switching. We also discuss the possible presence of two distinct switching mechanisms in Ag2S. Our findings could be extended to other semiconductor materials with mobile donors or acceptors.LEI Universiteit LeidenFOMQuantum Matter and Optic

Bulk and Surface Nucleation Processes in Ag2S Conductance Switches

We studied metallic Ag formation inside and on the surface of Ag2S thin films, induced by the electric field created with a STM tip. Two clear regimes were observed: cluster formation on the surface at low bias voltages, and full conductance switching at higher bias voltages (V > 70mV). The bias voltage at which this transition is observed is in agreement with the known threshold voltage for conductance switching at room temperature. We propose a model for the cluster formation at low bias voltage. Scaling of the measured data with the proposed model indicates that the process takes place near steady state, but depends on the STM tip geometry. The growth of the clusters is confirmed by tip retraction measurements and topography scans. This study provides improved understanding of the physical mechanisms that drive conductance switching in solid electrolyte memristive devices.Comment: In press for PR

Observing “Quantized” Conductance Steps in Silver Sulfide: Two Paralell Resistive Switching Mechanisms

014302Quantum Matter and Optic

Towards a quantitative description of solid electrolyte conductance switches

Quantum Matter and Optic

Single-Source Pulsed Laser Deposited Perovskite Solar Cells with > 19% Efficiency

Single-source vapor deposition of metal halide perovskites has, to date, remained challenging due to the dissimilar volatilities of the perovskite precursors, limiting the controlled transfer of multiple elements at once. This Chapter demonstrates that pulsed laser deposition (PLD) addresses the rate-control challenges of single-source evaporation, enabling solar cells with power conversion efficiencies (PCE) above 19%. For this, we combined dry mechanochemical synthesis and PLD to fabricate MA1-xFAxPbI3 and Cl-passivated MA1-xFAxPbI3 films from a single-source target. The films are grown onto hole-selective self-assembled monolayers (SAMs-2PACz), where first a thin PbI2-rich layer forms, leading to full perovskite conversion as confirmed by grazing-incidence wide-angle X-ray scattering. Onto the perovskite, an oleylammonium iodide (OAmI) post-treatment is then applied to passivate its top surface by forming a 2D perovskite film. This was followed via in-situ PL monitoring during the 2D application. Further, we found that when incorporating PbCl2 in the target and OAmI-based 2D passivation, a remarkable 19.7% PCE for p–i–n perovskite solar cells is achieved with enhanced device stability. These findings emphasize the importance of interface and passivation strategies to improve the performance of PSC-containing vapor-deposited absorbers. Further, these results represent one of the highest PCEs achieved within the state-of-the-art single-source vapor deposition methods, as far as our knowledge extends. Consequently, this study highlights the appeal of PLD to fully unlock the potential of single-source vapor-deposited perovskite towards low-cost and efficient photovoltaics

Toward Annealing Stable Molybdenum Oxide Based Hole Selective Contacts For Silicon Photovoltaics

Molybdenum oxide MoOX combines a high work function with broadband optical transparency. Sandwiched between a hydrogenated intrinsic amorphous silicon passivation layer and a transparent conductive oxide, this material allows a highly efficient hole selective front contact stack for crystalline silicon solar cells. However, hole extraction from the Si wafer and transport through this stack degrades upon annealing at 190 C, which is needed to cure the screen printed Ag metallization applied to typical Si solar cells. Here, we show that effusion of hydrogen from the adjacent layers is a likely cause for this degradation, highlighting the need for hydrogen lean passivation layers when using such metal oxide based carrier selective contacts. Pre MoOX deposition annealing of the passivating a Si H layer is shown to be a straightforward approach to manufacturing MoOX based devices with high fill factors using screen printed metallization cured at 190

Single-Source, Solvent-Free, Room Temperature Deposition of Black γ-CsSnI₃ Films

The presence of a non-optically active polymorph (yellow-phase) competing with the optically active polymorph (black $\gamma$-phase) at room temperature in CsSnI3 and the susceptibility of Sn to oxidation, represent two of the biggest obstacles for the exploitation of CsSnI3 in optoelectronic devices. Here room-temperature single-source in vacuum deposition of smooth black $\gamma$ - CsSnI3 thin films is reported. This has been done by fabricating a solid target by completely solvent-free mixing of CsI and SnI2 powders and isostatic pressing. By controlled laser ablation of the solid target on an arbitrary substrate at room temperature, the formation of CsSnI3 thin films with optimal optical properties is demonstrated. The films present a band gap of 1.32 eV, a sharp absorption edge and near-infrared photoluminescence emission. These properties and X-ray diffraction of the thin films confirmed the formation of the orthorhombic (B-$\gamma$) perovskite phase. The thermal stability of the phase was ensured by applying in situ an Al2O$_3$ capping layer. This work demonstrates the potential of pulsed laser deposition as a volatility-insensitive single-source growth technique of halide perovskites and represents a critical step forward in the development and future scalability of inorganic lead-free halide perovskites.Comment: Accepted by Advanced Materials Interfaces, 16 pages, 4 figures, and supplemen

APCVD of dual layer transparent conductive oxides for photovoltaic applications

We report the atmospheric pressure chemical vapour deposition (APCVD) of a dual layer transparent conductive oxide (TCO). This combines a fluorine doped tin oxide (FTO) base layer with a fluorine doped zinc oxide (FZO) top layer, where we seek to utilise the respective advantages of each material and the differences in their associated industrial deposition process technologies. Deposition of a 250 nm thick FZO layer on FTO was enough to develop features seen with FZO only layers. The crystallographic orientation determined by the FZO dopant concentration. Changes to the deposition parameters of the underlying FTO layer effected stack roughness and carrier concentration, and hence optical scattering and absorption. Photovoltaic cells have been fabricated using this TCO structure showing promising performance, with efficiencies as high as 10.21% compared to reference FTO only values of 9.02%. The bulk of the coating was FTO, providing the majority of conductivity and the large surface features associated with this material, whilst keeping the overall cost low by utilising the very fast growth rates achievable. The FTO was capped with a thinner FZO layer to provide a top surface suitable for wet chemical or plasma etching, allowing the surface morphology to be tuned for specific applications

Chalcohalide Antiperovskite Thin Films with Visible Light Absorption and High Charge-Carrier Mobility Processed by Solvent-Free and Low-Temperature Methods

Silver chalcohalide antiperovskites represent a rather unexplored alternative to lead halide perovskites and other semiconductors based on toxic heavy metals. All synthetic approaches reported so far for Ag3SI and Ag3SBr require long synthesis times (typically days, weeks, or even months) and high temperatures. Herein, we report the synthesis of these materials using a fast and low-temperature method involving mechanochemistry. Structural and optical properties are examined experimentally and supported by first-principles calculations. Furthermore, we deposit Ag3SI as thin films by pulsed laser deposition and characterize its optoelectronic properties using optical-pump-terahertz-probe measurements, revealing a high charge-carrier mobility of 49 cm2 V-1 s-1. This work paves the way to the implementation of chalcohalide antiperovskites in various optoelectronic applications